The conventional rolling bearing retainer consists of metal, polyamide resin, polyacetal resin or polybutylene terephthalate resin. Particularly, when a synthetic resin is adopted, only a synthetic resin which can be injection-molded is used or a synthetic resin composition composed of a molding material made of a synthetic resin and glass fibers, carbon fibers or organic fibers added to the molding material to reinforce the synthetic resin is used. To lubricate the rolling bearing having the retainer, a lubricating oil or a semi-solid lubricant such as a lubricating grease is used.
But when a large amount of the semi-solid lubricant such as the lubricating grease is used, owing to a resistance to stirring caused by the use of this lubricant, a torque for rotating a rotary shaft supported by the bearing becomes large, and a torque fluctuation during the rotation of the rotary shaft also becomes large. Particularly in a conventional retainer-provided rolling bearing lubricated with grease, when the rotational speed of the shaft (inner ring or outer ring of rolling bearing) becomes high, a torque required to rotate the shaft supported by the bearing becomes large owing to the resistance to stirring caused by the use of a large amount of the lubricating grease. Thereby the temperature of the bearing rises. Therefore the torque is liable to fluctuate or the lubricating grease is liable to leak. Further owing to the presence of the lubricating grease, a comparatively large amount of dust is liable to float on the periphery of the bearing.
To improve these problems, rolling bearing retainers allowed to have a lubricating function by impregnating the material thereof with a lubricant are proposed.
For example, the following bearing retainers are known: the bearing retainer composed of polyamide-imide resin, molded porously by compression molding, which is impregnated with fluorinated oil (patent document 1); the retainer composed of the molded oil-containing plastic, containing the oil-containing binder and the matrix, which is impregnated with the lubricating oil (patent document 2); the resin composition, consisting of the mixture of the polyolefin resin and the lubricating oil, which is molded in the shape of a retainer (patent document 3); the resin composition, consisting of the mixture of the synthetic resin, the fibrous oil guide material, and the lubricating oil, which is molded in the shape of a retainer (patent document 4); and the resin composition, consisting of the synthetic resin to which the porous silica impregnated with the lubricating oil is added, which is molded in the shape of a retainer (patent document 5).
But in the case of the bearing retainer composed of the polyamide-imide resin, molded porously by compression molding, which is impregnated with the fluorinated oil, after powdery resin is compression-molded, it is sintered. Thus as described later, it is theoretically and actually impossible that the interconnected hole porosity exceeds 30%. Therefore the amount of the lubricating oil capable of impregnating the polyamide-imide resin is less than the interconnected hole porosity of 30%. In dependence on a use condition, the lubricating amount may be short. Further there is a restriction in the kind of the resinous material in which an interconnected hole can be formed by the compression molding. That is, there is a restriction in the kind of the resin which can be used.
It is necessary that the retainer composed of the molded oil-containing plastic, containing the oil-containing binder and the matrix, which is impregnated with the lubricating oil is immersed in the lubricating oil at a high temperature (120° C. to 130° C.) for a long period of time (about seven days) to increase the oil impregnation amount. Therefore there is a fear that the lubricating oil and the resin forming the retainer deteriorate, and a large dimensional change is made. Consequently a product is inferior in stability, and it is difficult to keep a stable lubricity for a long time.
In the resin composition, consisting of the mixture of the polyolefin resin and the lubricating oil, which is molded in the shape of the retainer, the lubricating oil is held by the use of the polyolefin resin having a high oil-absorbing performance. Thus a small amount of the lubricating oil bleeds. Even though the lubricating oil is dispersed uniformly in the resin, it is technically difficult to bleed the lubricating oil from the inside of the retainer at a stable speed for a long time, although the lubricating oil in the neighborhood of the surface thereof.
When a large amount of the lubricating oil is used, a problem occurs in terms of production. For example, there occur disadvantages that a screw slides in injection molding; metering is unstably performed and hence a cycle time becomes long; dimensional accuracy is difficult to obtain; the lubricant sticks to the surface of a die and a molded surface has a poor finish; and the like. Therefore there is a limitation in the kind of the resin which can be used and in the kind of the lubricating oil which can be used, its viscosity, vapor pressure, and decomposition temperature.
In the resin composition, consisting of the mixture of the synthetic resin, the fibrous oil guide material, and the lubricating oil, which is molded in the shape of the retainer; and the resin composition, consisting of the synthetic resin to which the porous silica impregnated with the lubricating oil is added, which is molded in the shape of the retainer, the resin and the lubricating oil are mixed with each other before the resin compositions are molded. In this case, the lubricating oil is required to withstand a resin-molding temperature. Similarly to the above-described case, there is a limitation in the kind of the resin which can be used and in the kind of the lubricating oil which can be used, its viscosity, vapor pressure, and decomposition temperature. Because the screw slides in the injection molding, it is necessary to reduce the amount of the lubricating oil to stably supply the material into a molding machine. Because the maximum amount of the lubricating oil is less than 30%, the lubricating amount may be short in dependence on a use condition of the rolling bearing.
Inside the rolling bearing, usually, as the lubricant held inside the rolling bearing to decrease the friction coefficient between the inner and outer rings and the rolling element as well as the retainer and improve the durability of the bearing, a liquid lubricant, a semi-solid lubricant or a solid lubricant is appropriately used in dependence on the use thereof. For example, in a rolling bearing which is used in a closed clean atmosphere such as semiconductor-manufacturing equipment and particularly in a rolling bearing which is used in a low-pressure clean atmosphere such as a vacuum atmosphere, vapor generated from the liquid lubricant and the semi-solid lubricant and scattered fine particles may adversely affect the performance of precision parts. Therefore the liquid lubricant and the solid lubricant having a low vapor pressure are used. In recent years, because semiconductor precision parts and the like in which a conductive pattern has a very small width have come to have high performance, a very high dust-unraiseable performance is demanded.
Conventionally, as rolling bearings, for use in a vacuum apparatus, which comparatively favorably comply with the demand of the high dust-unraiseable performance, the retainer consisting of the porous article made of the polyamide-imide resin impregnated with the low vapor-pressure fluorinated oil used as the lubricating oil for the rolling bearing (patent document 1), and the retainer consisting of the porous article made of the polyimide, more heat-resistant than the polyamide-imide, which is impregnated with the fluorinated oil (patent document 6) are known. The bearing in which the retainer consisting of the porous article is impregnated with the alkylated cyclopentane oil serving as the lubricating oil (patent document 7) is also disclosed.
But in the rolling bearings of the patent documents 1 and 6, because the porous retainer is impregnated with the fluorinated oil serving as the lubricating oil, a large centrifugal force is applied to the retainer during a rotation of the bearing. Consequently the rotational efficiency of the bearing deteriorates, and the torque fluctuates to a high extent. These rolling bearings are not sufficiently reliable in the durability thereof when they are used at a high surface pressure (about 2 GPa).
In the bearing of the patent document 7, the above-described problem which occurs owing to the use of the fluorinated oil for the retainer is solved. But the interconnected hole porosity of the retainer is 5 to 25%. Therefore the amount of the lubricating oil with which the porous article of the retainer can be impregnated is small and hence it is impossible to prolong the period of time in which the bearing can be used.
The rolling bearing can be used for a food machine used to mix, knead, heat, dry, cool, charge, pack, and store food materials and edible products (or semi-products). As in the case of other machines, the bearing and other sliding parts are mounted on the food machine. It is necessary to prevent ingredients of these parts harmful to the human body from flowing into food. Therefore in accordance with the legal sanitary standard, it is necessary to choose materials such as resin, metal, lubricating oil, and grease, and additives composing the parts.
As the legal sanitary standard regarding various materials of the parts of the food machine, the authorization standards such as the standard of food and additives (notified by Ministry of Health, Labor and Welfare), FDA (Food and Drug Administration of U.S.), and H-I standard (standard that a material is unharmful to the human body when it directly contacts food) of USDA (Department of Agriculture of U.S.) are well known. These authorization standards specify ingredients which can be used as materials for the food machine, separately from materials for general industries.
In the known solid lubricant and rolling bearing for use in the food machine, the solid lubricant is used to prevent the lubricant from being washed away even though water penetrates into the bearing and make it difficult for rust to be generated even though a solution of salt penetrates into the bearing to maintain the lubricating properties of the bearing and the like for a long time (patent document 8).
The rolling bearing for use in the food machine is known in which the solid lubricant, for the food machine, which is not washed away with water and withstands a successive use at a high temperature higher than 150° C. is enclosed in the bearing to make it difficult for rust to be generated in a condition in which the bearing contacts a solution of salt (patent documents 9 and 10).
In these rolling bearings for the food machine, although the solid lubricant is not washed away with water, the solid lubricant is produced by the method of kneading the resin and the lubricant in advance to obtain the greasy resin and thereafter calcining the mixture, with the resin enclosed in the bearing. Thus even in the combination of the resin and the grease both of which can be used at a high temperature as disclosed, the resin is calcined at a high temperature. Therefore there is a possibility that the lubricating oil deteriorates while the resin is being calcined. Consequently in putting the bearing to practical use, there may be a case in which restrictions are imposed on the combination of the resin and the grease both of which can be used at a high temperature. Thus the solid lubricant has a problem that the degree of freedom in the combination of the resin and the grease suitable for a use is low (patent documents 8 to 10). Further to prevent the bearing from being rusted, a large amount of the solid lubricant is increased in the bearing. Therefore the bearing has a problem that it has a high torque (patent document 8) until the solid lubricant is compatible with the bearing.
Patent document 1: Japanese Patent Application Laid-Open No. 61-6429
Patent document 2: Japanese Patent Application Laid-Open No. 1-93623
Patent document 3: Japanese Patent Application Laid-Open No. 8-21450
Patent document 4: Japanese Patent Application Laid-Open No. 11-166541
Patent document 4: Japanese Patent Application Laid-Open No. 2002-98152
Patent document 6: Japanese Patent Application Laid-Open No. 8-177866
Patent document 7: Japanese Patent Application Laid-Open No. 10-169661
Patent document 8: Japanese Patent Application Laid-Open No. 10-36875
Patent document 9: Japanese Patent Application Laid-Open No. 11-335687
Patent document 10: Japanese Patent-Application Laid-Open No. 2001-131569